The subject invention relates to the field of strain measurements. Examples of applications which can benefit from the subject technology include engineering analysis of strain on complex geometries, design analysis, and non destructive testing of structures. Accordingly, the subject invention can be utilized in the automotive, aerospace, civil structures, and sporting goods fields, as well as many others. Specific embodiments of the subject invention pertain to a strain-sensitive coating, a strain field mapping system, and a method for conducting strain analysis. The method and apparatus of the subject invention are particularly useful in the field of full-field strain analysis.
In the field of structural analysis, the ability to determine of the stresses which a structural body is experiencing can provide important feedback in the design and construction of such structural bodies. Typically, surface strain on the structural member provides information on the stresses that the body is experiencing. This information leads to the identification of stress concentrations, over stressed areas, and general stress mapping for design and comparison to predictive methods. Currently, a number of methods exist for measuring such surface strain, including point and full-field methods.
The point methods include electrical resistance strain gauge methods and methods employing electro-optic sensors and optical methods. These methods typically require the affixing of a plurality of sensors at various locations on a structural body, or stepping the sensor across the structural body, such that when the structure is stressed each sensor or step indicates the surface strain at one point. In order to determine the strain over an entire body, numerous sensors located at critically stressed points on the surface are required or numerous steps are required for movable sensors. Accordingly, these point methods can be cumbersome, making it difficult to determine the stresses over an entire surface of a structure.
A number of surface measurement techniques have been developed to overcome the limitations of the point methods, including brittle coatings, photoelastic coatings, moire methods, interferometric methods, and digital image correlation methods. Each of these methods can be useful for certain applications but have characteristics which limit their usefulness. Brittle coatings typically provide good qualitative information about the principal stress directions on objects. Some limitations of brittle coatings are that the part can only be tested in one loading configuration, it only provides limited quantitative information, and methods for automated data collection do not exist. Photoelastic coatings provide only the shear stress and principal stress direction information on objects and are typically cumbersome to apply to large bodies since the coating process is time consuming. Moire methods are typically limited to flat objects and are not used on complex geometries. Interferometric methods such as holographic interferometry, electric speckle pattern interferometry and shearography require sophisticated vibration isolation greatly reducing their applicability. Digital image correlation methods lack the sensitivity required to test parts in the material linear range.
Accordingly, there still remains a need for a method and system which can easily and accurately measure full field strain on complex shaped structures. Even more advantageous would be a method and system which can provide real-time dynamic strain measurements, even at low strain levels.